Electron tube circuits



May 31, 1932- D. F. WHITING ELECTRON TUBE CIRCUITS Original Filed Nov.21, 1924 3 Sheets-Sheet -1 May 31, 1932. D. F. WHITI G ELECTRON TUBECIRCUITS Original Filed NOV. 21, 1924 3 Sheexs-Sheet 2 i UT lnrenfor:vDonald/7' W/zifirrg.

by Alfjl.

May 31, 1932. D. F.- WHITING ELECTRON TUBE CIRCUITS 3 Sheets-Sheet 3Original Filed Nov. 21, 1924 1,114,152,510 lllllllll i lnmnfor 00/70/00.I'M/77779.

Patented May 31, 1932 UNITED STATES PATENT OFFICE DONALD F. WHITING, OFPORT WASHINGTON, NEW YORK, ASSIGNOR T0 WESTERN ELEC- TRIO COMPANY,INCORPORATED, OF NEW YORK, N. 'Y., A CORPORATION OF NEW YORK ELECTRONTUBE CIRCUITS Application filed November 21, 1924, Serial No. 751,202.Renewed January 22, 1831.

This invention relates to space discharge devices and particularly tocircuits for energizing or controlling such tubes.

One object of this invention is to prevent fluctuations in the spacecurrent supply of a space discharge amplifier from producingdisturbances in the output of the amplifier.

A further object of this invention is to prevent fluctuations in spacecurrent supply from being impressed on the grid through the use of aresistance in the plate circuit for obtaining a grid biasing potential.

It is often desirable to energize electron tubes from alternatingcurrent sources such as house lighting circuits. In such cases it isnecessary to rectify the alternating current so as to obtain a directcurrent for the anodecathode voltage. The output of the rectifier can besmoothed out to a considerable extent by the use of a simple inductancecondenser filter. When smoothed out in this manner the supply voltage isusually sufficiently free of fluctuations that it can be used in thelast stages of an am lifier without producin undesirable distur ances inthe output. owever, the power level in the first stages is so lowparticularly when very weak currents are being amplified, that smallfluctuations, which may be present in the output of the usual smoothingout filter, may produce disturbances in the output of the first stageswhich are of the same order of magnitude as the amplified current.

In the types of vacuum tube apparatus which it is desirable to supplywith rectified current, it is also usually desirable to eliminate allbatteries, and the grid biasing potential must then be obtained in somemanner such as by the use of a resistance in the space, current circuit,the grid being connected to the terminal of the resistance adjacent tothe negative terminal of the space current source in a manner wellknown. in the art. By this arrangement, a potential proportional to theRI drop in the resistance, is impressed on the grid. If there are anyfluctuations in the space current supply the potential impressed on thegrid will fluctuate in the same manner and the fluctuations will beamplified by the action of the tube producing disturbances in theoutput.

It is a particular object of this invention to eliminate fluctuations inthe plate and rid potentials supplied to the tubes in the rst sta es ofan electron tube amplifier.

Iii general, the objects of this invention are accomplished by usingresistance condenser filters. Since there is no direct current drawn bythe grid of the tube, it is possible to include a high resistance in thecircuit connecting the grid with the impedance used for obtaining thebiasing potential. This high resistance will offer a considerableimpedance to fluctuatin currents and by using a condenser of mo eratecapacity connected in shunt to the circuit comprising this resistanceand the grid biasing resistance in series, it has been found that it ispossible to prevent flue? tuations in the supply voltage from beingimpressed on the grid.

Resistance condenser filters are also provided in the space currentsupply circuit in addition to the usual inductance condenser filter.Such filter stages are adapted for use in the supply to the firststages, particularly when these sta es are operated at lower voltagesthan the fast stages. In such cases the resistance elements of thefilter can be chosen so as to reduce the voltage to the desired value.Additional potentiometer filter stages of this type can also be providedfor reducing and further purifying the grid biasing potential for thetubes of the first stage.

It has also been found that electrolytic rectifiers employin filmforming plates are well suited for rectif yingpower supply especially asthey may be provided with extra plates, which, because of the highcapacity between the plates and the electrolyte, are very well adaptedfor use as shunt elements of the smoothin -out filter.

In one fiorm, this invention comprises a two sta e amplifier employingtwo tubes in a pus -pull circuit in the last stage and a single lowvoltage filament tube in the first stage. Power is supplied to theamplifier from an alternating current source, the filaments being heatedwith alternating current and the current being rectified for the platevoltage supply. The supply to the tubes of the last stage is filtered bymeans of an inductance condenser filter. Resistance and condenserelements are arranged in the supply circuit to further filter and reducethe voltage supplied to the tube of the first stage. The grid biasingpotential for the last stage is obtained by connecting a resistance inthe plate current circuit and connecting the grids through a resistancecondenser filter to the terminal of the resistance adjacent the negativeterminal of the space current supply. An additional resistance condenserfilter stage is included to reduce and purify the potential supplied tothe grid of the tube of the first stage.

This invention will be more clearly understood by reference to thefollowing description in connection with the drawings in I which Fig. 1shows a two stage transformer coupled amplifier embodying certainfeatures of this invention.

F ig. 2, a three stage resistance coupled amplifier embodying otherfeatures of the invention.

Fig. 3, a. two stage amplifier employing two tubes in push-pull circuitin the last stage and embodying still other features of this invention.

Fig. 4, a modification of the two stage amplifier circuit of Fig. 1, and

Fig. 5, a power supply circuit for electron tubes comprising an aluminumplate rectifier and a multi-stage smoothing out filter employingelectrolytic condensers as shunt elements.

In Fig. 1 a two stage amplifier is shown employing two three-electrodetubes 6 and 7 which are coupled together and to the input circuit 8 andoutput circuit 11 by means of the inter-stage transformer 10, inputtransformer 9 and output transformer-12. The tubes are energized fromthe alternating current source 13, power being supplied through thetransformer 14, the primary winding 15 of which is connected to thesource. The cathodes are heated with alternating current supplieddirectly from the secondary winding 16, the connections from thecathodes to the grid and plate circuits being made at the mid-point ofthe secondary winding much in the same manner as is disclosed in theWhite Patent No. 1.195.632 granted August 22, 1916, so as to preventundue disturbances in the output circuit 11 due to the alternatingcurrent supplied to the filaments. A rectifier 17 which may be, forexample, a two electrode electron discharge rectifier as-is shown,supplies plate current to the two tubes. The output of the rectifier issmoothed out by means of a filter comprising a series inductance 18 andtwo shunt condensers 19 and 20. The path of the plate currents for thetubes 6 and 7. for example. may be traced from the anodes of the twotubes, through the primary windings of transformers l0 and 12respectively, inductance 18, secondary winding 21 of the transformer 14to the cathode 22 of the rectifier 17 and through the space in therectifier to the anode 23, through the resistance 24 to the groundconnection which is connected to the mid-point of the secondary winding16 of transformer 14 and thence to the cathodes of the two tubes 6 and7. The grids of the two tubes 6 and 7 are connected through thesecondary windings of the respective transformers 9 and 10 andresistance 25 to the terminal 26 of resistance 24 adjacent to the anode23 of rectifier 17, which is the negative terminal of the plate currentsource. In this manner the grids are maintained at negative potentialswith respect to the cathodes of tubes 6 and 7 by means of the RI drop inthe resistance 24. The large capacity condenser 27 serves to shunt theamplified currents around the resistance 24 so as to reduce the couplingbetween the input and' output circuits of the tubes. If only a singlecondenser was used in this manner it would have to be of very highcapacity. However in addition to this element there are included, theresistance 25 and the condenser 28 which serve to further reduce thecoupling between the input and output circuits. to a value so small thatit does not interfere with the operation of the amplifier. By making thevalue of resistance 25 very high the capacities of the condenser can bereduced to very nominal values. Since there is no direct current drawnby the grids of the tubes there is no direct current potential acrossthe resistance 25 and the value of this resistance is not limitedthereby. It has been found that very good filter action may be securedby setting the value of the resistance 25 at about the order of onemegohm and using as large capacity condensers for 27 and 28 as isconvenient.

This resistance condenser filter not only serves to reduce the couplingbetween the output and input circuits of the tube but also preventssmall fluctuations in the RI drop in the resistance 24, due to ripplesin the output of the rectifier, from being impressed on the grids oftubes 6 and 7 and being amplified thus producing objectionabledisturbances in the output current of the amplifier.

Referring to Fig. 2 which shows a three stage resistance coupledamplifier, the three electrode-electron discharge tubes 30, 31 and 32which are coupled together and to the input and output circuits throughthe usual circuit arrangements. The direct current battery 33 is shownfor supplying the space current to all of the tubes, though this may bereplaced by a rectifier arrangement similar to that shown in the otherfigures. In

addition to the resistance and condenser ele-' ments 3st, 35, 37 and 33which correspond respectively to the elements it, '25. 27 and 28 of thecircuit of Fig. 1, there are included the resistances 3t) and 40 whichconstitute a potentiometer to obtain grid potentials for the first twotubes and a czuracity ll, which ctmperates with the resistance 30 tostill further reduce the disturbing potential which otherwise would heapplied to these grids. In this ca e there is a small potential drop inthe resistance because of the current drawn by the potentiometer butthis drop may be made negligibly small. if the resist ulu'cs 3S) and 40have values of the order of one m'cgohni and resistance 35 is about50,000

connects the first tube to the incoming cir- 'llllt 8. The usualinter-stage and output transformers 5t and couple the two tubes 51 and52 to the first stage tube 50 and to the output circuit 11 in the usualmanner for push-pull operation.

Tubes in the amplifier are energized from an alternating current source,power being supplied to a transformer 56, the primary winding 57 ofwhich is arranged to be connected to an alternating current source suchas a house lighting circuit through a switch and an attachment plug 59.The filaments of the tubes are heated with alternating current, beingconnected directly to the secondary winding 60 in a similar manner tothat shown in the circuit of Fig. 1. The winding 60 is shielded by meansof a grounded shield 61 in order to prevent longitudinal currents fromaffecting it and making the amplifier noisy. Since the amount of noisein the plate circuit of a vacuum tube due to the filament being heatedby alternating current is a function of the filament voltage and sinceany disturbance in the plate circuit of the first stage tube 50 will beamplified by the tubes 50 and 51 before appearing in the output circuit11, a low voltage filament tube is used in the first stage. This tubehas a filament voltage which is preferably at least as low as one volt.Filament current for tube 50 is, therefore, tapped oil the secondaryWinding 60 at different points than those at which the filament currentfor the tubes 51 and 52 is tapped off.

A rectifier 62 supplies space currents to the tubes. The. filament 63 ofthistube is heated by alternating current supplied from the secondarywinding (34. The alternating current which is applied to this tube tofurnish a rectified current is provided by an additional secondarywinding 65. Filtering of this rectifier current is accomplished in thefollowing manner. A condenser 67 is provided to complete the alternatingcurrent circuit which includes the high voltage secondary winding (35and the rectifier tube 62. This condenser shunts the circuit into whichthe rectified current flows, thereby reducing the alternating potentialapplied to it. A retardation coil further prevents the alternatingcurrent from flowing in the path which the rectified current takes.Another condenser Gt) reduces still further the alternating currentpotential applied to the plates of the tubes of the push-pull stage. Thedirect potential applied by therectifier to the plates of tubes 51 and52 is reduced to the somewhat lower potential applied to the tube 50 bymeans of a potentiometer consisting of two fixed resistances 70 and 71.A condenser 72 is connected to shunt the resistance 71 to afford abypass by means of which the alternating current in the plate circuit ofthe tube 50 may return to the filament after passing through the primarywinding of the inter-stage transformer 54. This condenser performs theadditional function of still fur-' ther filtering any residualalternating current from the rectified current.

The grid biasing potential for the amplifier tubes is secured by passingthe entire rectified current through a resistance 73. In order toeliminate any residual alternating current hum which may be present inthe voltage drop across this resistance, and in order to prevent anycoupling between the output and input circuits which may not becompletely eliminated by the condenser 69, the resistance 74: andcondenser'75 are employed, in much the same manner as the elements 25and 28 of the circuit of Fig. l, to filter the voltage applied to thegrid. The potential drop across the resistance is reduced to a suitablevalue for applying to the grid of the tube 50 by means of apotentiometer consisting of resistances 7 6 and 77. As was the case inthe circuit of Fig. 2, the resistance of this potentiometer should behigh in order that the direct current flowing through the resistance 74will be low and consequently cause very little loss of voltage becauseof resistance drop. Another condenser 78 connected in shunt to theresistance 77 still further filters the voltage applied to the grid ofthe tube 50 thus reducing the residual hum which may remain in spite ofthe use of the condenser 75, and it also provides a low inipedanceby-pass to the filament of tube 50 for charging currents to the grid ofthis tube. It should be noted in this connection that the condenser 67is connected in such a manner as to shunt the alternating currentflowing through rectifier tube 62 away from the resistance 73 thusrendering the voltage developed across this resistance more free fromfluctuations than if this condenser had been connected to ground. On theother hand, the condenser 69 is connected to ground thereby shuntingaway from the resistance 73 any voltage surges in the plate circuit ofthe push-pull stage which would occur if the stage were overloaded andwhich might otherwise result in blocking the amplifier and rendering ittemporarily inoperative.

Amplification control is obtained by making the connection 79 to thesecondary winding of the input transformer 53 variable. An equalizernetwork 80 is also shown connected in shunt to the primary of the inputtransformer 53 for making the resultant amplification and reproductioncharacteristic of the amplifier and loud speaker respectively moreuniform. The efl'ect of this network is controlled by means of the key81. \Vhen the key is actuated the network has the effect of making theamplification characteristic of the amplifier uniform over a very widerange of frequencies. lVhen the key is in its normal position, that is,in the position shown, the resistance connected in series with theequalizer is reduced thereby raising the amplification in theneighborhood of 140 cycles and, to a slight extent, in the neighborhoodof 5000 cycles, while reducing it considerably in the middle frequencyrange. An amplification characteristic of this type adapts the amplifiervery well for use with certain horn type loud speakers which normallytend to distort the sound considerably in the opposite manner.

The low filament voltage tube 50 may be, for instance, of the typecommonly known as the 215A and the tubes 51 and 52 may each be of the21GA type, in which case it has been found desirable to rectify a totalcurrent of 33 milliamperes of which 30 milliamperes supplies the platesof the tubes 51 and 52, a space current of 15 milliamperes being normalfor each under these conditions. One milliampere is the normal spacecurrent of the 215--A vacuum tube under the conditions imposed and theadditional two milliamperes are lost in the potentiometer which is usedfor supplying the tube 50 with plate voltage. In this circuit asatisfactory capacity value for the condensers 67 and 69 is 2 m. f., forthe inductance 68, 15 henrys, resistances 70 and 71, 48,000 ohms and30,000 ohms respectively, the condenser 72, 2 m. f., a value of 565 ohmsfor the resistance 73 gives a grid biasing potential for the tubes 51and 52 of 18 volts, in which case the filter resistance 74 and condenser75 may be 48,000 ohms and m. f. The grid potential for tube 50 should bereduced to 4% volts by means of the potentiometer comprising theresistances 76 and 77 for which the values 1 megohms and megolnnrespectively, are satisfactory while the condenser 78 may have acapacity of m. f.

The circuit of Fig. 4 is the same as that of Fig. 1 except that anelectrolytic rectifier 83 is used for rectifying the plate currentsupply. This rectifier consists of two aluminum plates 84 and 85 and athird electrode 87, which may be composed of lead, tin, carbon or otherinert conductor, immersed in an electrolyte which may be for example asolution of ammonium borate. Such an arrangement will permit current topass from the lead plate but never from the aluminum plates so that whenthe aluminum plates 84 and 85 are connected to the terminals of thesecondary winding 86 rectified current may be drawn from the midpoint ofthe secondary winding 88 and the plate 87 which serve as the positiveandnegative terminals respecti ely and this current can be passed throughthe smoothing out filter in the manner shown. 7

Fig. 5 shows a power supply circuit for the electron tubes 90, 91, 92and 93 which may be used in an amplifier circuit, for example. The leadsto the grids and plates are shown dotted to indicate the omission of theconnections by which the tubes are associated in the circuit in whichthey are employed.

Power is supplied through the transformer 94 which has its primarywinding 95 connected to the alternating current source 96. The filamentsof the tubes are heated by means of alternating current being connectedto the secondary winding 97 of the transformer 94. Connections from thecathodes to the grids and plates are made at the midpoint of thesecondary 97 in a similar manner to that in the circuit of Fig. 1.

An electrolytic rectifier 98 which operates in a similar manner to therectifier of Fig. 4 supplies plate current to the tubes. In this theelectrolyte is contained in a tin tank 99 which serves as the anode. Theoxide coating which forms on the surface of the aluminum plates 101 and102 and by virtue of which the rectifier functions also forms thedielectric for a high capacity between the plates 103 and 104 and theelectrolyte. Thus, by including extra plates in the bath, high capacitycondensers for use in the smoothing out filter may be obtained. In thecircuit shown two plates 103 and 104 are used. These are connected ateach side of the retardation coil 105, which is connected in series withthe lead from the midpoint of the secondary winding 100. The dottedcapacities 106 and 107 represent the capacity between the plates 103 and104 and the electrolyte, which capacities shunt the circuit into whichthe rectified current flows thus reducing the alternating currentpotential applied to that circuit.

The point 108 is connected to the plate of the tube 93 for supplying theplate voltage. The rectifier output is then further filtered by means ofthe inductance 109 and additional electrolytic condenser elements. Theseelements may be formed by using more plates in the same bath, thoughthis method is found to have certain disadvantages.

Since the rectifying plates 101 and 102 normally receive rectifiedcurrent impulses flowing to them from tank 99 by the way of theelectrolyte contained within and surrounding them, the rectifier circuitbeing completed through plate 103 and capacity 106 to the electrolyteand back to the tank, and since in flow ing through the electrolytethese currents cause potentials to be established in the directions ofthe paths taken by these currents, owing to the resistance of theelectrolyte to such currents, and that these potentials cause potentialgradients to be established in all of the electrolyte surrounding theseplates, it is apparent that any plate immersed in such an electrolytewill have impressed upon it from the electrolyte a potential which isequal to the average potential existing over that area of theelectrolyte in which the plate is immersed. As long as this straypotential is small with respect to the potential im-' pressed upon theplate from the circuit, as in the case of the first section of a filter,this stray potential causes little effect; but if the stray potential.becomes appreciable with respect to the potential impressed upon it'fromthe circuit, as in the case oij succeeding sections oi the filter, thestray potential will have deleterious effects often to the extent ofrendering these succeedin filter sections ineffectual. It is thereforedesirable to mount the additional condenser plates in a separate tank.111 this case additional filtration is provided by means of a plate 111mounted in the tank 110. This electrolytic condenser and the retardationcoil 109 serve to further filter the current supplied to the plate ofthe tube 92.

The plate current for the tubes 90 and 91 is filtered by means of theresistance condenser filter stages comprising the resistances 114 and115 and the plates 112 and 113. The resistances 11 1 and 115 are soproportioned as to reduce the plate voltages to the desired values. Thegrid biasing potentials are obtained by an arrangement similar to thatof the circuit of Fig. 2. A resistance 134 which corresponds to theresistance 34 of the circuit of Fig. 2 being included in the rectifiedcurrent circuit and the IR- drop in this resistance being impressed onthe grids of the tubes 92 and 93 after being filtered by the action ofthe condensers 137 and 138 and the resistance 135 in a manner similar tothat of the circuit of Fig. 2. Similarly the potential for the grids ofthe tubes 90 and 91 is reduced and further filtered by the use of theresistances 139 and 140 and the condenser 141 which corresponds to theelements 39, 40, and 41 of Fig. 2.

While this invention has been described in connection with specificcircuits embodying its features it should be understood that it is notlimited to these specific circuits and particupreceding stage, saidsecond mentioned tube being adapted to operate at a lower anodecathodepotential than said first tube, a source of alternating currentpotential, a transformer, a primary connected to said source, and anassociated secondary, said secondary having atap at the mid-point,connections from said secondary to the cathodes of said tubes forsupplying heating current of the proper potential thereto, a rectifierhaving its input terminals connected to said source, an inductanceconnected to the positive terminal of said rectifier, a condenserconnected between the terminal of said inductance remote from saidrectifier and the mid-point of said secondary, a connection from theabove mentioned terminal of said inductance to the anode of said firstmentioned tube, a resistance havin one terminal connected to the abovementioned terminal of said inductance and the other terminal connectedto the anode of said second mentioned tube, an impedance connectedbetween the negative terminal of the rectifier and the mid-point of saidsecond ary winding, a connection from the terminal of said impedanceadjacent the negative terminal of said rectifier to the controlelectrode of said first mentioned tube, said connection including a highresistance element, a condenser connected between the terminal of saidresistance element remote from said impedance and the mid-point of thesecondary winding, a potentiometer connected in shunt to said condenser,a connection from a tap on said potentiometer to the control electrodeof said second mentioned tube, and a condenser connected in shunt to theportion of said potentiometer between said tap and the mid-point of saidsecondary winding.

2. In a multi-stage amplifier comprising a plurality of electron tubes,a source of alternating current, an electrolytic rectifier comprising afilm forming aluminum electrode and non-film forming electrode immersedin the electrol e, a connection from the positive terminal 0 therectifier to the anode of one of said tubes said connection including animpedance element, another film forming aluminum plate immersed in theelectrolyte and connected to one terminal of said impedance element, aconnection from the positive terminal of said rectifier to the anode ofanother one of said tubes, said connection including a second impedanceelement, and an electrolytic condenser comprising a film formingaluminum plate connected to the above mentioned terminal of said secondimpedance element and immersed in an electrolyte contained in a separatecontainer from that of the rectifier and a non-film forming plateconnected to the negative terminal of the rectifier.

8. In a multi-stage vacuum tube amplifier circuit. adapted to besupplied from a source of rectified alternating current. common circuitmeans comprising series impedances aml shunt capacities for obtainingsuccessively higher positive potentials for the anodes of said vacuumtubes and to eliminate noise currents from said anode circuits arisingin said source. and common circuit means comprising other seriesimpedances and shunt capacities for obtaining successively highernegative potentials for the control electrodes of said vacuum tubes andto eliminate noise currents in said control electrode circuits.

4. A multi-stage vacuum tube amplifier in accordance with claim 3 inwhich said source of rectified current comprises electrolytic rectifiermeans rectifying an alternating current supply, said means containingelectrodes forming certain of said shunt capacities.

5. A current and voltage supply network for a plurality of vacuum tubesin tandem comprising a series resistance having its terminals connectedto a source of voltage, a common connection from the resistance to thecathodes ot the tubes, tap points on the portions of said resistanceincluded between said common cathode connection and the positiveterminal of the resistance for supplying successively higher values ofanode voltage to the successive tubes. other. tap points on the portionof the resistance on the other side of the common cathode connection forsupplying succcssively higher values of negative biasing voltage to thegrids of the successive tubes, an individual condenser connected betweenthe common eathode connection and each of said tap points and a filtermeans connected across the entire resistance. said condensers and filtermeans serving to suppress noise components arising from the supplysource.

6. In a multi-stage amplifier having a plurality of electron tubes, eachhaving a thermionic cathode, an anode, and a control electrode connectedin a plurality of stages, a source of rectified alternating current, aresistance having a terminal connected to the negative terminal of saidsource and an other terminal connected effectively to the cathodes, saidresistance lying in the path of the rectified current, a secondresistance in shunt to said first resistance and large compared to thefirst resistance, a connection from the control electrode of one of saidstages to a point in said shunt resistance, a connection from thecontrol electrode of a stage subsequent to the first mentioned stage toa point in said shunt resistance intermediate the first mentioned pointand the negative end of theresistauce whereby the control electrode ofthe. subsequent stage receives a larger negative bias than that of theother stage, and individual condensers connected between the said pointsof connection to said shunt resistance and the cathodes T. In amulti-stage amplifier having a plurality of electron tubes connected ina plu ralitv of stages, one of said tubes being adapted to work at alower anode-cathmie potential than another of said tubes in a subsequentstage. a connection between the. cathodes of said tubes, :1 rectifiertor supplying space current to said tubes. a smoothing-out filtercomprising an impedance in ser es and a plurality of condensers in shuntto the path of the rectified cuirent at dili'ercnt points on saidimpedance for filtering the space current to said tubes, connectionstrom the output of said filter to the anode of said last mentioned tube,and to said common cathode connection. a potentiomet r connected acrossthe output of said filter and having a tap connected to the anode ofsaid first mentioned tube. the shunt capacity provided by saidcondensers together with the resist aucc of the portions of saidpotentiometer between the connections to said athode connection and tosaid respective anodes serving to eliminate from the circuits otsaidanodes the noise currents arising in said rectifier, an impedanceconnected between said common cathode connection and the negativeterminal of said rectifier and connections from ditferent points of saidimpedance for supplying different negative grid potentials to saidtubes, said impedance and shunt condensers providing a smoothing-outfilter to eliminate from the circuits of said control electrodes thenoise current arising in said rectifier.

In witness whereot. I hereunto subscribe my name this 14th day ofNovember A. D. 192-

